1. Field of the Invention
The present invention relates to an image forming apparatus in which constant current control is performed to a transfer voltage used to transfer a toner image borne by an image bearing member or an intermediate transfer member to a recording material, particularly to a method for setting a constant voltage of a lower limit value of the transfer voltage to which the constant current control is performed.
2. Description of the Related Art
An image forming apparatus in which the toner image formed on the image bearing member is transferred to the recording material passing through a transfer portion and an image forming apparatus in which the toner image transferred to the intermediate transfer member is transferred to the recording material passing through the transfer portion are widely used.
In the image forming apparatus, the toner image is transferred to the recording material by applying the transfer voltage to the transfer portion in which a transfer member abuts on the image bearing member or the intermediate transfer member, and the transfer voltage that is of a constant voltage determined according to a predetermined transfer current value is generally applied to the transfer portion. This is because, as described later, when the constant current control is performed to the transfer voltage, a current passed through the outside of the recording material or a portion in which the toner image does not exist on the recording material makes a value of a current passed through the necessary toner image uncertain (see FIGS. 4A and 4B). Also, when the constant voltage is applied, the transfer current can be secured in the necessary toner image according to the constant voltage irrespective of the current passed through the outside of the recording material or the portion in which the toner image does not exist on the recording material.
However, a transfer member constituting the transfer portion largely varies according to a variation of a product, a member temperature, and an accumulated use time, and a resistance value of the recording material passing through the transfer portion varies according to a kind of the recording material, environmental humidity, and a humid state of the recording material.
Therefore, when the constant voltage control is performed to the transfer voltage, electric characteristics (such as resistance value) of the transfer member are measured to adjust the constant voltage in starting up the image forming apparatus or at the beginning of continuous image formation.
For example, Japanese Patent Application Laid-Open No. 2004-117920 discloses an image forming apparatus in which a constant voltage is applied to a transfer member to transfer a toner image borne by an intermediate transfer belt to a recording material. In this case, a predetermined voltage is applied to a transfer portion in which the recording material does not exist immediately before the continuous image formation is started, a current value is taken to obtain a voltage value corresponding to a predetermined target current, and a recording material divided voltage according to the kind of the recording material is added to the voltage value to set the constant voltage.
However, because a transfer member resistance value gradually decreases by temperature rise during the continuous image formation, the constant voltage that is proper immediately before the continuous image formation is started possibly to become improper. Also, when a humid state varies in each recording material even if the same kind of the recording material is delivered, the recording material resistance value changes, whereby the constant voltage that is proper in the first recording material possibly becomes improper in the 50th recording material.
Therefore, there is proposed a technique of re-measuring the transfer member resistance value in each ten sheets during the continuous image formation to re-adjust the constant voltage. However, a delivery interval between the recording materials expands to lower productivity of the continuous image formation, or image density changes in a stepwise manner before and after the re-adjustment. The technique therefore cannot deal with the variation of the humid state in each recording material at all.
On the other hand, when the constant current control is performed to the transfer voltage applied to the transfer portion, all the recording materials are automatically adjusted to the transfer current, in real time during the continuous image formation, according to the transfer member and the recording material resistance value. Therefore, the productivity of the continuous image formation is not lowered, the image density does not change in the stepwise manner, and the variation of the humid state in each recording material can be dealt with.
Japanese Patent Application Laid-Open Nos. 10-48965 and 11-288184 discloses an image forming apparatus in which a voltage to which constant current control is performed is applied to a transfer member to transfer a toner image borne by an intermediate transfer belt to a recording material. In this case, in order to avoid the drawback (the current value passed through the portion in which the toner image exists becomes uncertain) of the constant current control, a lower limit value is set to the voltage applied in the constant current control. The constant voltage of the lower limit value is therefore applied to the transfer member in the range where the voltage applied to the transfer member becomes lower than the lower limit value.
In the control disclosed in Japanese Patent Application Laid-Open No. 10-48965, the constant voltage is applied to the transfer member in the range where the voltage applied in the constant current control becomes lower than the lower limit value, so that at least a certain current value passed through the portion in which the toner image exists can be secured even if the recording material absorbs moisture to extremely lower the resistance value.
However, it was turned out that because the lower limit value of the voltage applied in the constant current control is a fixed value, the lower limit value becomes gradually improper when the transfer member resistance value increases continuously according to the accumulation of use time, and finally the lower limit value does not fulfill the function. It was turned out that the range to which the constant voltage is applied moves to the outside of the variation range of the recording material resistance value, and the constant voltage is not applied to the recording material that absorbs the moisture to extremely lower the resistance value (see FIGS. 5A and 5B).